I. Field of the Invention
This invention relates to the measurement of temperatures in molten aluminum and aluminum alloys. More particularly, the invention relates to a method and apparatus for protecting temperature measuring devices, such as thermocouple devices, from molten aluminum and aluminum alloys, and to protected temperature measuring devices suitable for immersion into molten aluminum and aluminum alloys.
II. Discussion of the Prior Art
In processes and equipment for handling molten aluminum and aluminum alloys, metal temperatures must frequently be measured. This is needed, for example, in metal melting furnaces, in-line degassers, filter boxes, and other equipment associated with metal holding and casting operations.
These applications require the use of measuring devices, normally thermocouple devices, that are not only protected from attack by molten aluminum and aluminum alloys for the desired period of measurement, but that are also protected from mechanical damage in the plant prior to installation in the equipment, and that are also protected from the high thermal shock that occurs when the devices are inserted directly into molten aluminum (a sudden temperature change of up to 700.degree. C. can occur upon such insertion). A fast temperature response is also highly desirable.
Frequently, temperature is measured using a thermocouple device positioned inside a larger steel or cast iron tube, coated with a metal-resistant coating or wash as a protective layer. Such devices offer good response time, but once the protective layer fails, either through mishandling in the plant, or by abrasion within the molten metal vessel, the steel or cast iron tube rapidly fails, and the thermocouple fails shortly thereafter. Such devices are inexpensive, but the requirement for frequent replacement offsets the low unit cost.
Some alternative metals may be considered for the protective tube, but almost all common metals dissolve in molten aluminum and aluminum alloys and experience premature tube failure. Thermocouple devices themselves have been constructed using a metal outer sheath and inner refractory powder layer to protect the thermocouple element. Such devices are described, for example, in U.S. Pat. No. 3,942,242 (Rizzolo). However, once the metal outer sheath fails, the powder cannot prevent metal penetration to the measuring element, and furthermore, the powder is released to contaminate the molten metal. In addition, the extensive use of powder insulation reduces the thermal response of the device.
Thermocouple assemblies have also been constructed with metal outer sheaths and solid refractory or ceramic inner sheaths. A device of this kind is described in U.S. Pat. No. 5,071,258 (John D. Usher et al.). However, the refractory or ceramic is chosen for strength and toughness rather than thermal shock resistance, and therefore tends to crack on insertion into molten aluminum or aluminum alloy. Furthermore, many refractories are reactive with molten aluminum and its alloys; therefore, once the metal outer sheath fails, the inner layer does not provide protection and the entire assembly fails. When such assemblies fail, the refractory pieces are heavier than the molten aluminum or alloy and are not easily removed. In vessels such as in-line degassers, the refractory fragments can damage rotary injectors during subsequent operations.
Thermocouple assemblies intended for short term use have also been constructed with an end cap that is designed to fail but that provides temporary protection for the tip of the assembly when the assembly is passed through dross or flux present at the metal surface. Such a device is disclosed, for example, in U.S. Pat. No. 4,358,630 (Richard A. Falk). However, such devices are designed to be removed from the molten metal after a short period of time (and then refurbished with a new protector cap). Disadvantageously, the protector cap does not offer protection from general mechanical damage, nor from mechanical damage elsewhere than at the tip of the device.
Accordingly there is a need for an inexpensive, thermal shock resistant, temperature measuring device, particularly a thermocouple assembly, offering good mechanical protection prior to use, long term protection from molten aluminum and aluminum alloys when in use, good thermal response, and ease of cleanup when the assembly eventually fails.